An active-matrix substrate for use in a liquid crystal display device and other devices includes switching elements such as thin-film transistors (which will be hereinafter referred to as “TFTs”), each of which is provided for an associated one of pixels.
As for TFTs, a TFT which uses an amorphous silicon film as its active layer (and will be hereinafter referred to as an “amorphous silicon TFT”) and a TFT which uses a polysilicon film as its active layer (and will be hereinafter referred to as a “polysilicon TFT”) are used extensively.
Recently, people have proposed that an oxide semiconductor be used as a material for the active layer of a TFT instead of amorphous silicon or polysilicon. Such a TFT will be hereinafter referred to as an “oxide semiconductor TFT”. Since an oxide semiconductor has higher mobility than amorphous silicon, the oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. Also, such an oxide semiconductor film can be formed by a simpler process than a polysilicon film.
An active-matrix substrate including TFTs as switching elements is called a “TFT substrate”. On a TFT substrate, the drain electrode of each TFT is connected to its associated pixel electrode. A transparent conductive film of ITO (indium tin oxide) or IZO (indium zinc oxide), for example, is generally used to make the pixel electrodes. Their source and drain electrodes, as well as source lines, are generally formed out of the same conductive film. As the conductive film, a film with a high degree of conductivity such as an aluminum (Al) film is suitably used. However, if the Al film were allowed to contact with the semiconductor layer of a TFT, Al would diffuse inside the semiconductor layer too much to achieve intended TFT performance in some cases. In addition, in a configuration in which the Al film is allowed to contact with ITO or any other material that makes the pixel electrodes, there is so significant a difference in ionization tendency (or standard electrode potential) between Al and ITO that Al could get corroded electrolytically during the wet etching process of ITO and the contact resistance might rise eventually. Such a problem arises not only when an Al film is used but also when another metal that could cause the electrolytic corrosion reaction to a transparent conductive material such as ITO is used as an electrode or line material.
To overcome such a problem, Patent Document No. 1 teaches using a stack including an Al film and a barrier metal film as source and drain electrodes.
FIG. 9 illustrates a part of the cross-sectional structure of the TFT substrate disclosed in Patent Document No. 1.
In the TFT substrate disclosed in Patent Document No. 1, each TFT 190 includes a gate electrode 123, a gate insulating film 140, a semiconductor layer 154 of amorphous silicon, and source and drain electrodes 173, 175. The source and drain electrodes 173 and 175 are electrically connected to the semiconductor layer 154 via contact portions 163 and 165, respectively. The TFT 190 is covered with a protective film 180. A pixel electrode 185 of ITO or IZO, for example, is formed on the protective film 180. The pixel electrode 185 is in contact with the drain electrode 175 inside a contact hole formed in the protective film 180.
In the TFT substrate of Patent Document No. 1, the source and drain electrodes 173 and 175 each have a multilayer structure including a lower film 173p, 175p of a barrier metal and an upper film 173q, 175q of aluminum or an aluminum alloy. The upper film 173q, 175q is removed partially so that the lower film 173p, 175p is exposed at both ends of the semiconductor layer 154. The pixel electrode 185 is in contact with the lower film 175p of the drain electrode 175 inside the contact hole.
According to the configuration shown in FIG. 9, the lower film 173p, 175p of a barrier metal is interposed between the upper film 173q, 175q of Al and the semiconductor layer 154, and therefore, Al can be prevented from diffusing so far as to enter the semiconductor layer 154. In addition, since a portion of the upper film 175q of Al is removed from a region where the pixel electrode 185 and the drain electrode 175 are connected together, it is possible to prevent contact between Al and ITO from causing electrolytic corrosion of Al.